1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, a heat treatment apparatus, and a heat treatment method using the apparatus. Specifically, the invention relates to a heat treatment method and a heat treatment apparatus for crystallizing an amorphous semiconductor film, for recrystallizing and activating a semiconductor film that has been brought back to an amorphous state by ion implantation or ion doping, and for gettering of a metal element remaining in the semiconductor film. The invention also relates to a semiconductor device manufacturing method using this heat treatment method.
2. Description of the Related Art
Having silicon as its main ingredient, a crystalline semiconductor film formed at a process temperature of 600° C. or lower is often called low temperature polysilicon. The primal use of this crystalline semiconductor film is of as an active layer for forming a channel formation region, a source or drain region and the like in a thin film transistor (hereinafter referred to as TFT). The TFT is formed on a glass substrate to manufacture a liquid crystal display device, and technologies related thereto are receiving attention most.
Technologies for manufacturing a TFT using the above crystalline semiconductor film characteristically employ laser annealing and ion doping, and these techniques make it possible to form an n-channel TFT and a p-channel TFT on a large area glass substrate to build an integrated circuit having the CMOS structure.
A TFT needs, in addition to an impurity region of n type or p type conductivity to serve as a source and drain region, a low concentration impurity region that serves as LDD (lightly doped drain) useful in reducing leak current and in stabling TFT characteristics. The TFT also needs to be doped with an impurity element of one conductivity type in order to control the threshold voltage. Formation of the low concentration impurity region and the doping with an impurity element of one conductivity type are conducted through ion doping and subsequent activation treatment. In this ion doping, all of ion species generated are implanted by acceleration implantation without being subjected to mass separation.
The impurity is activated after doping by furnace annealing, laser annealing, or RTA (rapid thermal annealing). The source and drain region, which is relatively heavily doped (concentration: 1015 atoms/cm2), is said to need a rather high temperature and take a longer time to activate the impurity element used in doping for enhancement of conductance. Laser annealing melts a semiconductor film and is unsatisfactory in controllability and reproducibility, which makes it difficult to employ laser annealing in a mass production process. On the other hand, furnace annealing is suitable for a mass production process since it treats in batches but its activation efficiency decreases as the process temperature becomes low to prolong the treatment time, which is a problem.
Laser annealing as a crystallization technique is capable of forming a crystalline semiconductor film on a glass substrate. However, the reaction is of nonequilibrium one and the crystals formed therefore have small grain size and many defects. Laser annealing crystallization has little direct control factors other than the energy density and irradiation number of laser light and the substrate heating temperature. Furthermore, the direct control factors are effective only for limited ranges. For example, control by energy density is effective when it is 250 to 400 mJ/cm2 and it only gives an amorphous structure outside the range.
A crystallization technique involving adding with a metal element is disclosed in Japanese Patent Application Laid-open No. Hei 7-183540 as a method that can provide better crystals than those obtained by laser annealing. The metal element used is nickel, palladium, lead, or the like. Various methods can be employed in the doping, such as plasma treatment, evaporation, ion implantation, solution application, and sputtering. Crystallization is made by heat treatment at 500 to 600° C., preferably 550° C. for four hours. However, this method leaves the metal element in the semiconductor film crystallized and often needs gettering. Most of the remaining metal element forms a deep level in the forbidden band of the semiconductor film to act as a lifetime killer and to cause an increase of leak current in the junction.
Gettering using phosphorus can make the metal element segregate in a region doped with phosphorus. This gettering uses an annealing furnace and requires heat treatment typically at 450 to 600° C. for twelve hours. The metal element thus segregates in the phosphorus-doped region.
One of the most promising application fields for the thus manufactured TFT is liquid crystal displays and other flat panel displays. In the flat panel display field, increasing the substrate size is demanded for improvement of productivity. These larger substrates come in various sizes and 960×1100 mm2 is among them. A substrate measuring 1000 mm or more in one side is more often considered than others. Liquid crystal display devices are not the only ones who face this demand but it is a common object to all large-area integrated circuits constructed from TFTs formed on glass substrates.
In order to improve the productivity, it is necessary to reduce the number of steps in a TFT manufacturing process and shorten the treatment time. Then, a furnace annealing apparatus, which treats in batches, would not be helpful in improving the production efficiency. If the furnace annealing apparatus is enlarged, current consumption is increased for heating the large capacity furnace, not to mention it needs larger installment area.
RTA is suitable in terms of productivity. RTA is capable of heating and raising the temperature high in a short period of time and latently has higher processing ability than furnace annealing also when the single wafer method is employed. However, short heating time means high heating temperature while a temperature exceeding the distortion point of glass, or even its softening point, is required in order to obtain desired effects in activation and gettering. For instance, a glass substrate is bent and deformed by its own weight by merely sixty seconds of heat treatment at 800° C. for the purpose of gettering.